The fractions of the title relate to those existing in the interval between photo-sensitive elements of the type used in a goniometer for measuring characteristic angles of motor vehicle wheels.
In these types of goniometers, the angle measured is the angle formed with the reference axis of the goniometers (which is perpendicular to the optical sensor and which defines an origin for the measurement of the distances in a longitudinal direction) by a light beam which is trained on the optical sensor by an optical device.
The light beam is constituted by parallel rays which are generated by a light located at a sufficiently large distance with respect to the distance between the optical device (cylindrical lens or slit) and the linear sensor.
When a cylindrical lens is used as the optical device, the focal length of the lens is equal to the distance between the lens and the sensor. In this case all the parallel rays which strike the front surface of the lens are concentrated in a line which intersects the linear sensor in a very precise zone which is conditioned not only by the exactness of the lens positioning but also by the angle to be measured which is the angle formed by the incident light beam with the line perpendicular to the linear sensor. It is obvious that as the angle becomes the greater so do the alterations of the precisely-directed light hitting the sensor, with the immediate consequence that altered images are produced by the sensor, resulting in a progressive increase in the lack of precision of the measurement.
The phenomenon is progressively more accentuated the smaller the focal distance and the greater the angle to be measured.
One way of reducing this phenomenon is to use a corrector optical group in association with the sensor. This however leads to considerable complications in construction as well as higher costs.
A further consideration is that the resolution of the linear sensor is physically connected to the distance between one photosensitive element and an adjacent one.
This means that according to known realizations it does not seem possible to determined a distance from the origin which is not equal to a whole multiple of the interval (constant) between element and element. Thus it does not seem possible to read fractions of this interval.
A similar imprecision would occur should a light beam be collimated with a transversal dimension able to generate a light spot which is smaller than the dimensions of a photosensitive light element. The result could be that the linear sensor may not even be activated in all those cases where the light beam did not strike any of the photosensitive elements.
The possible solution, of increasing the threshold of sensitivity by increasing the number of photosensitive elements per unit of length of the linear sensor, or by considerably reducing the interval between one photosensitive element and another, is at present so expensive as to be impracticable.
In any case a greater goniometer resolution for measuring the characteristic angles of a motor vehicle""s wheels using linear sensors together with a greater width in the field of measurement is an established need in the field.
The main aim of the present invention is to obviate the limitations and drawbacks in the prior art.
An advantage of the invention is that it does not introduce any special modifications, from the constructional point of view, to the apparatus used. These aims and advantages and others besides are all achieved by the present invention, as it is characterized in the claims that follow.
In a process for reading fractions of an interval between contiguous photo-sensitive elements in a linear optical sensor, of a type used in a goniometer, an angle measured is an angle formed with a reference axis of the goniometer, perpendicular to the linear optical sensor, by a light beam which is trained on the optical sensor by an optical device. The process comprises: a reading of a current image constituted by an ordered totality of intensities of incident radiations read on contiguous photosensitive elements; processing of data taken from the current image by means of a process which converges towards a result defining, with respect to an original determined by an intersection of the reference axis with an axis of the sensor, a distance d of a point of incidence on the sensor of an ideal optical axis of the light beam.